The present invention relates generally to power converters and, more particularly, to a method and apparatus for controlling a single-phase converter or the zero-sequence circuit of a three-phase converter.
A high control gain at the steady-state operating point (e.g., 60 Hz fundamental frequency) is desirable for power converters in order to minimize control error and to enhance dynamic performance. For a DC/DC converter, an infinite control gain at its steady-state operating point can theoretically be achieved using a proportional-integral (PI) compensator. In three-phase systems, a conventional control approach is to perform a DQ (Direct-Quadrature) transformation. The three-phase stationary coordinates are converted into DQ rotating coordinates so that the balanced three-phase sinusoidal waveforms in stationary coordinates become a DC value in rotating coordinates. Thus, not only is the DC operating point obtained for control design and stability analysis, but also a PI voltage or current compensator in rotating coordinates gives an infinite control gain at the fundamental frequency. Therefore, control accuracy at the fundamental frequency is easily achieved. Essentially, the DQ transformation converts the three-phase circuits with sinusoidal waveforms in stationary coordinates into several DC/DC converters with DC waveforms in rotating coordinates. Although DQ transformation and controller design in rotating coordinates are simple and powerful with respect to improving performance at the fundamental frequency, it is not directly applicable to single-phase power converters. That is, the DQ transformation needs balanced three-phase variables or two orthogonal rotating variables as inputs in order to achieve DC steady-state operation. Of course, for single-phase power converters, there is only one phase available.
For single-phase power converters, e.g., inverters and power factor correction rectifiers, the variables are sinusoidal in steady state. The design of controllers to achieve high control gains at their steady-state operating points is thus particularly difficult due to the time-varying voltages and/or currents.
In one approach, the design of a controller for single-phase power converters is performed in the same way as that of a DC/DC converter, i.e., ignoring the sinusoidal wave shape of either the reference voltage for inverters or the input voltage and reference current for PFC rectifiers. This control design approach, which follows the classic linear time-invariant system theory, is somewhat problematic since there is no true DC operating point due to the alternate current operation of the single-phase power converters. Typically, a so-called xe2x80x9cquasi-steady-statexe2x80x9d design approach is adopted wherein a few points along the sinusoidal operating waveform are selected as moving DC operating points. The controller and closed loop system stability are checked at the selected quasi-steady-state DC operating points. At least two problems arise with this approach: (1) The control design process does not reflect the true sinusoidal steady-state operating condition, and thus the control gain varies in a line cycle; and (2) The control gain at the fundamental frequency is limited to ensure stability, causing a significant control error in both the voltage/current amplitude and phase, unless the power converter operates at a very high switching frequency. In order to damp LC filter resonance in a single-phase inverter and thus improve control bandwidth, various multiple loop controllers have been proposed using the same linear time-invariant system control design approach, including capacitor current feedback, inductor current feedback, and load current feedforward, and variations thereof.
Another approach has been to use a nonlinear control structure, wherein the output of a linear voltage or current compensator that controls the RMS value of the voltage or current, is multiplied by a sinusoidal template to provide control to the power stage. This approach has two drawbacks. First, the control loop gain continuously varies along the sinusoidal waveform template, and it is lowest at the zero crossing, causing significant distortion. Second, the performance under nonlinear load is much worse as a result of multiplication in the controller, giving rich harmonic contents due to the frequency modulation effect.
Yet another approach is to use a high Q bandpass filter at the fundamental frequency in lieu of the traditional integrator in a PI compensator. However, this is difficult to implement with either analog or digital means and does not provide insight into stability margin.
Accordingly, it is desirable to provide an apparatus and control for single-phase power converters to achieve high control gain at the fundamental frequency. It is furthermore desirable to apply DQ transformation to single-phase power converters with implementation and stability analysis in rotating DQ coordinates, in order to achieve an infinite control loop gain at the fundamental frequency.
Imaginary orthogonal circuit state variables are established to enable transformation of real circuit state variables of a single-phase converter or the zero-sequence circuit of a three-phase converter from stationary coordinates to DQ rotating coordinates, thereby transforming the sinusoidal steady-state operating point of the real circuit into a DC operating point. A control provides the imaginary circuit state variables in a shifted queue comprising memory blocks 34 for storing quarter-cycle shifted real circuit variables. Control of the converter is thus implemented in rotating DQ coordinates to achieve a theoretically infinite control loop gain at the fundamental frequency.